Genetic linkage and color polymorphism in the southern platyfish (Xiphophorus maculatus): a model system for studies of color pattern evolution.

Color pattern polymorphisms are widespread in animals, and are found within populations, among populations, and among species. The southern platyfish, Xiphophorus maculatus, represents one of the most extreme examples of color pattern polymorphism. Extensive research with this model system for melanoma formation has resulted in an understanding of the underlying genetic basis of over 40 sex-linked and autosomal color patterns, including alleles that code for melanin, pterin, and carotenoid coloration. Research has also found that genes that affect sex determination, timing of sexual maturation, and coloration are genetically linked on the sex chromosomes. In many animals, color patterns often show strong sexual dimorphism, with conspicuous coloration limited to males. Although some of the color pattern alleles are sex-limited in platyfish, many are expressed by both sexes. Despite the abundance of work on this model system, little is known about the evolutionary processes responsible for this diversity of color pattern alleles. This review discusses what is known about platyfish coloration, and the roles that genetic linkage and variation in environmental conditions within and among populations might play in the evolution and maintenance of the extreme color pattern polymorphism exhibited by this platyfish.